BAGHDAD — The culmination of the long, harder-than-expected battle to drive Islamic State from Iraq is at hand.

Following weeks of steady but bloody progress, Iraqi government forces announced Thursday that they were close to recapturing the landmark Nuri mosque in Mosul, a hugely symbolic victory retaking the holy site where Islamic State leader Abu Bakr al-Baghdadi made his only known public appearance in 2014 and from which he declared the establishment of a radical Islamic “caliphate.”